FORM-REGULATION IN CERIANTHUS. 1 05 



apparent for the permanent reduction of the tentacles after a cut 

 near the oral end and the absence of any such effect after a cut 

 near the middle. The difference in the distance between cut and 

 tentacles in the two cases is certainly not sufficient to justify the 

 conclusion that in the one case a shock or stimulus of some sort 

 reaches the tentacles while in the other case it does not, nor can 

 it serve as a basis for explanation if the change be regarded as 

 simply osmotic in nature, not as a reaction to a stimulus. 



If, however, we consider the difference in the relations of parts 

 in the two regions of the body and the effect upon the internal 

 pressure all difficulty disappears. If the lateral cut be made at 

 any of the levels indicated in Fig. 9 it will simply enter the in- 

 termesenterial divisions of the enteron which open axially into 

 the central cavity everywhere aboral to the oesophagus, since the 

 axial margins of the mesenteries hang free in the cavity. There 

 is nothing with which either margin of the cut can unite except 

 the other margin. When cuts at these levels close they can 

 close only by means of union of the cut edges. It tollows that 

 the same relations between the part oral to the cut and the other 

 regions of the body are not altered by the cut. 



The immediate consequence of such a cut is loss of water from 

 the enteron and complete collapse of body and all tentacles. In- 

 rolling of the body-wall adjoining the cut soon begins and ap- 

 proximation of the inrolled portions is brought about by the 

 elasticity of the other parts of the body-wall especially that of 

 the opposite side of the body (Child, '04^, Figs. 8 and 9). Oc- 

 casionally when the cut is not far below the aboral end of the 

 oesophagus, the contraction of the body-wall and of the thick- 

 ened mesenterial margins and filaments in consequence of the cut 

 almost or quite closes the intermesenterial chambers between the 

 cut and the aboral end of the oesophagus. It follows that when 

 water again enters the enteron through the oesophagus other re- 

 gions will become distended, but these will not and consequently 

 the corresponding tentacles will remain collapsed. At this stage 

 the internal pressure is always much lower than that in normal 

 animals since increase of the pressure beyond a certain point 

 causes the approximated margins of the cut to separate and the 

 water is lost. The point to which I wish to call attention, how- 



